[X86][SSE] combineX86ShuffleChain - combine INSERT_VECTOR_ELT patterns to INSERTPS

Noticed while trying to cleanup D66004 - if a shuffle operand came from a scalar, we're better off using INSERTPS vs UNPCKLPS as this is more likely to load fold later on. It also matches our existing BUILD_VECTOR lowering.

We can extend this to other PINSRB/D/Q/W cases in the future as the need arises.